The present invention relates generally to the manufacture of holographic substrates or holograms, and more particularly to the manufacture of holograms of images by manipulating laser beams to reflect image data.
Holograms composed of gratings formed by light interference patterns generated using coherent monochromatic light from the laser are used in a variety of applications. One such application is the provision of holograms as identity or security devices on credit cards. In order to provide holograms on large numbers of credit cards, it is necessary that a master copy of the hologram be made so that multiple copies can be duplicated in a process similar to that of xe2x80x9cstamping outxe2x80x9d records. One example of a procedure for the conversion of a photograph into a hologram is found in U.S. Pat. No. 3,832,027. This patent includes a disclosure describing a procedure in which a two-dimensional photograph is converted into a hologram by means of computer data processing. Multiple views of the photograph are aggregated to provide the ultimate view which image upon which the holographic gratings constituting the hologram are formed.
Additional background regarding the manufacture of holograms is found in an article entitled xe2x80x9cDiffraction Gratingsxe2x80x9d published at page E-29 of the 1984 edition of the xe2x80x9cOptical Industry and Systems Purchasing Directoryxe2x80x9d. This publication describes the manufacture of holographic gratings by the interference of two beams of coherent monochromatic light altering a photosensitive material which is used as the master for turning out copies of the hologram. The interference fringes of the two light beams are formed where the two coherent light beams come together to cancel or reinforce the peaks and valleys of each of the beams. These interference fringes result in a physical altering of the photosensitive material so that a series of grooves are formed. The ultimate image formed by reflecting light from the holographic gratings is determined by the spacing of those gratings.
Another method for making a chromatic holographic image is found in U.S. Pat. No. 4,498,729. The method includes the steps of making a monochromatic hologram on a first photographic plate, then making a diffraction grating by exposing a second photographic plate to a series of co-linear point sources of mutually coherent monochromatic light. Then the exposed plate is developed and bleached to produce the diffraction grating. A second hologram is made by exposing a third photographic plate to an image from a narrow elongated strip of the first hologram with the diffraction grating in the optical path. The achromatical ray is made by holographically recording the image produced by eliminating the second hologram with monochromatic light on a fourth photographic plate.
Because of the complexity of handling multiple overlapping images computer aided holography such as that disclosed in U.S. Pat. No. 4,778,262 has been required in the conventional art for precise holography. In this patent, an illumination model is provided to specify sources of light rays and dispersion particles of the object. Each light ray being specified by a path and an intensity function is traceable from a source via the object to a set of points and space by the computer. The hologram is synthesized from a plurality of smaller hologram elements. Each individual element sustains a field of view of the object. The light rays from the object line within the field of view and along the lines of sight are sampled by the computer. Optical means are employed to physically reproduce the sample light rays using coherent radiation. The reproduced coherent light rays are then interfered with a coherent reference beam to form the hologram element. In the alternative, the hologram elements are calculated using a computer. Using this technique, the holographic surface is logically partitioned into a grid within the computer, where the contribution of light from the object to each grid element is envisioned as a bundle of light rays emanating from each part of the object and converging onto each grid element. The intensity of each ray of light arriving at a given grid element is determined by the computer by tracing the light ray from its source to the associate part of the object and then onto the grid element in accordance with the given illumination model. Thus, a xe2x80x9ctreexe2x80x9d of light rays, each in terms of direction and intensity is generated for each grid element. Since the illumination model can be manipulated on the computer, the rendering of the object can easily be modified. This enables complicated lighting of the object not readily practical by physical means. The entire hologram is synthesized by forming, in turn, the hologram element at each grid element on the holographic surface. This is done by either reproducing the associated xe2x80x9ctreexe2x80x9d of light rays associated with a predetermined grid using coherent radiation and made to interfere with a coherent reference beam, or simulating the same on the computer.
Multiple exposures of images is a necessary expedient in the conventional art. As indicated in U.S. Pat. No. 3,615,123, a holographic system for recording multiple-exposure holograms requires the use of a pulsed laser. Using this technique, each repetitive reference beam is deflected so that it reaches the recording material at a plurality of discrete different angles. Each of the multiple-exposure recordings of the hologram may be produced by a reference beam having the same angle as that at which it was taken. A reference beam may, for example, be deflected by utilizing an electro-optical retarder followed by a birefringent crystal for deflecting the beam in accordance with its direction of polarization. The paths of the reference beams for each discrete angle may be equalized by the provision of a plurality of reflectors disposed along in ellipse having the recording material in a first reflector as its focal points.
In order to distinguish and phase and amplitude differences, the system of U.S. Pat. No. 4,212,536 used a technique of holographic subtraction with phase modulation. In this technique, two substantially identically patterned transparencies, i.e., a master photo mask and a copy thereof are compared with each other by transluminating the master with an object beam, producing a hologram of that master by letting the object beam interfere with a reference beam from a common source of coherent light such as a laser. The copy is placed in the path of the object beam formerly occupied by the master and the developed hologram is positioned at the intersection of the two beams to generate a compound beam of zero intensity if the two transparencies are identical. The luminous energy of the compound beam thus varies inversely with the degree of equivalency of the two transparencies. The compound beam may be imaged onto a receiving surface with certain areas thereof blocked out to eliminate error indications from marginal zones.
The conventional technology also encompasses the use of holograms for information storage. In U.S. Pat. No. 4,111,519, a bite of binary of data to be stored is recorded as a synthetic Fourier transformed hologram of the bite of data. A time varying control signal representing the synthetic hologram is used to intensity modulate a coherent light beam as the beam scans transversely across a photosensitive recording film. The amplitudes of different spatial frequencies in the data band are differently altered to compensate for signal-to-noise roll-off over the data band. The thus compensated signals are also modified to compensate for non-linear gain characteristics. The thus modified signals are also processed to provide the modulating control signal. During the read-out process, an inverse Fourier transformation is performed optically on the light diffracted by the synthetic hologram in order to produce the original bite of data in the form of an optical intensity pattern. This pattern is transversely distributed on an array of photosensitive detectors which converts the optical intensity pattern to an electrical data signal. During this read-out process, a tapered neutral density filter, i.e., a wedge filter, is employed in front of the photosensitive detectors to provide attenuation or gain across a spatial frequency bandwidth to compensate for system modulation transfer function.
Data along the Z axis is also encompassed by the conventional art as indicated by U.S. Pat. No. 4,498,740. In this system, a hologram is written from X, Y and Z data by representing an information beam at a holographic medium with X and Y coordinates represented by X and Y position on the medium and Z coordinates represented by distance between the holographic medium and a point position of the beam close to the medium, while simultaneously presenting a reference beam in interference with the information beam. The size of the reference beam is comparable to the size of the information beam at the holographic medium. In this system, the area of the information beam and the reference beam at any position on the holographic medium is a small fraction of the total area of the hologram.
Conventional art techniques are able to transfer only limited amounts of image information (or other data) into the control of the process for forming diffraction gratings. Consequently, there are severe limitations as to the clarity and accuracy of the images that can be produced from conventionally made holograms.
One object of the present invention is to provide an accurate pixel-by-pixel representation of a copied image on a hologram constituted by diffraction gratings.
Another object of the present invention is to provide a system for quickly and accurately conveying image information on a pixel-by-pixel basis to a holographic grating.
Yet another object of the present invention is to provide a system for making holograms in which the image is adjusted based upon the angle of a viewer with respect to the hologram.
Still a further object of the present invention is to provide a system in which the apparent position of an image for a viewer in a predetermined position is based upon pixel playback angle.
These objects are accomplished according to the present invention in which positional data (using an X-Y coordinates system) and image data for each pixel of a plurality of pixels representing an image is obtained, stored, and used to control an apparatus for making a hologram composed of diffraction gratings. The apparatus is controlled so that a laser beam is split into a reference beam and at least one object beam used to represent image data for a corresponding pixel. The reference beam and other beam(s) are recombined at a photoresist material to form an interference pattern. The angle of the object beam(s) with respect to the photoresist surface and the duration of the object beam(s) interfering with the reference beam are used to convey data regarding the corresponding pixel. Additional pixel data can be conveyed by multiple exposures of the single object beam (with the reference beam) or by exposures of a plurality of object beams for each pixel. A movable table is used to move the photoresist from one pixel location to another in accordance with the X-Y coordinate system by which the pixel locations of the original image were determined.
Data in the Z-axis direction including the apparent position of an image reflected from the hologram can be adjusted by adjusting the playback angle of xe2x80x9csteroscopicxe2x80x9d pixel pairs when forming the diffraction gratings constituting the pixels.